The present invention relates to an improved air bag mounting system. More specifically the invention is directed to an arrangement for retaining a preassembled state among the various components of an air bag module to insure the integrity of such subassembly during final assembly operations. Among the components of the air bag module are the cover, air bag cushion, air bag clamping means (annular retainer, module housing and fasteners), gas generator or inflator, ignition element and a means to attach the assembly to the body of the automobile.
The inflator, due to the presence of the gas generant, a pyrotechnic, is the most sensitive component. Accordingly, for safety reasons it is desirable to delay installation of the inflator to as late as possible in the assembly process. Thus, the housing, air bag cushion and annular retainer are usually preassembled as a subassembly before installation of the inflator. It is important that the aforementioned three components of the air bag subassembly be firmly held together so that they will not become dislodged prior to final assembly.
In a conventional mounting for an air bag, the peripheral region around the outer circumference of the gas inlet opening into the air bag is clamped between a module housing provided around the gas discharge or exit opening and an annular retainer secured to the module housing. The module housing is mounted to the vehicle body, for example, the steering wheel. The annular retainer is bolted or riveted to the module housing.
Both riveting and bolting add another step in the manufacturing process, which serve to delay production by increasing the time required to produce an air bag subassembly. Further, workman sometimes spend an inordinate amount of time trying to line up the corresponding holes for insertion of the bolts and/or rivets.
The use of rivets is also a major cause of reworking of the inflator module assembly due to the necessity of replacing broken or unfastened rivets. Reworking of the inflator is not always possible. Riveting sometimes poses the danger of damaging the air bag during the handling and riveting operations thereby compromising the structural integrity of the air bag module. In these situations the defective modules must be discarded, resulting in lost production and increased cost. The elimination of rivets in the manufacture of the air bag module assembly obviates a major cause for reworking and/or discarding of defective air bag modules.
Air bag deployment in the event of a collision takes place in a time period of approximately one-twentieth of a second and the large forces produced thereby tend to pull the clamped portion of the bag from the module housing. Therefore, it is important that the retaining ring be able to firmly hold the air bag in place so as to prevent a large amount of blow-by of the inflating gas into the passenger compartment of the vehicle. Currently gases from the inflator escape into the passenger compartment through discrete vents by design. A small amount of blow-by would not add a large amount of gas to the passenger compartment. Blow-by is typically undesirable due to the effect it has on module components and air bag cushion fill.
One solution to provide clamping of the air bag and to increase the shear strength of the air bag between the fastener and the inner edge of the gas inlet opening is to dispose a plurality of slotted studs perpendicularly away from the axial face of the retainer ring as disclosed by Hartmeyer, U.S. Pat. No. 4,988,119. The slotted studs have enlarged heads which deflect due to the central slot when passed through an aperture in the air bag and base plate and expand once through the aperture to engage the material surrounding the aperture. Rivets are then disposed through the studs to further engage the base plate.
Another solution is described in U.S. Pat. No. 5,176,400 to McGuire et al, wherein an annular retainer ring is provided with projecting pins which form a frictioned interference fit with apertures in the module housing. The projecting pins are forced into the apertures forming a frictioned interference fit capable of holding the air bag subassembly together. In another embodiment the projecting pins are "peened over" to form a clamping action. Another prior art construction is disclosed in U.S. Pat. No. 5,141,247 to Barth which describes a retainer ring having tabs which are bendable into position for exerting clamping forces on the air bag, inflator and support plate.
Co-pending application Ser. No. 08/048,567 filed Apr. 15, 1993 and assigned to the assignee of the present invention discloses a similar annular retainer ring comprising axial descending tab means. The tabs after passing through apertures in the air bag cushion and module housing can be bent or deformed so as to engage the underside of the module housing thus clamping the air bag cushion to the housing.